Hydraulic lift mechanism



2 Sheets-Sheet 1 Nw., 11.3, 1945. sfB. MCLEOD HYDRAULIC LIFT MECHANISMFiled July 16, 1940 S. B. MCLEOD HYDRAULIC LIFT MECHANISM Nov. 13, 1945.

Filed July 16, 1940 2 Sheets-Sheet 2 WN wk Mum www d. ZH@ M f m5. mi m wZ 5MM Patented Nov. 13, 1945 UNITED sTATEs' PATENT oFFlcT-: :saam I'smarty* Intranets Mien.,eon onanermemm applicants .my 1s, me, seran No.34am' sown. (cnet- 52) This invention relates to a hydraulic liftmechanism or system for raising and lowering or opensively, to theraising and lowering of so-called col lapsible tops for vehicle bodiesof the convertible s type One of the objects of the present invention isto provide simplified mechanism of the foregoing character capable ofeasy application to or installation in a vehicle body, and one in whichample and easily controlled power is provided for the raising orlowering movement of the top.

Another object of the invention ls to provide improved mechanism of theforegoing character which is relatively compact and capable ofinstallation in a relatively small space.

A further object of the invention is to provide an improved hydrauliclift mechanism including an operating cylinder and piston structurehaving a single piston rod, and means for compensating for thedifference in volume or space within the cylinder at opposite sides ofthe piston, said means also compensating for volume changes of the fluidin the system due to the operating pressures and expansion orcontraction of the fluid due to temperature variations and permittingmanual actuation of the top without using the pump.

Another object of the invention is to provide an improved hydraulic liftmechanism or system having means which permits the system to be illledwithout the necessity of providing for the external bleeding thereof.

Another object of the invention is to provide improved hydraulic liftmechanism which may include an airtight and upper compensating reservoirand a fluid system connected therewith which together constitute asealed unit.

Other objects and advantages of this invention electric motor, fluidpump, valve mechanism.

associated parts, embodying the present invention: and

Fig. 4 is a view similar to Fig'. 2, showing a modified form ofvalvemechanism embodying the presentinvention; and s Fis. 5 is a view similarto Fig. 4 showing the.'

valves in their neutral leaking position and also showing anothermodified form of valve unit embodying the present invention.

Before explaining in detail the present invention it is to be understoodthat the invention is not limited in its application to the details ofconstruction and arrangement ofparts illustrated in the accompanyingdrawings, since the invention is capable of other embodiments and ofbeing practiced or carried out in various ways. Also it is to beunderstoodthat the phraseology or terminology employed herein is Vforthe purpose of descnption and not of umsehen.

Referring now particularly to Fig. 1, there is shown one adaptation ofthe present invention. In the present instance the hydraulic lift mecha-5 nism or system of the present invention is shownas installed in therear deck compartment or space Il of a motor vehicle body of theconvertible type in which the top is adapted to be raisedand lowlered toprovide a closed or open body. Ihe mechanism shown may be mounted in anyconvenient location, such as upon the floor I I of the rear deck vcompartment' or in a channel member of the ve- 4 hicle frame, partsthereof being located below and thus concealed by the floor.

In the present embodiment, the lift mechanism is attached or connectedto the linkage or frame l member. such as the member I2 of thecollapsible top for the vehicle body, and comprises a cylinder Il havinga piston Il mounted upon a single piston rod Il which is preferablyconnected with the will appear in the following description and ap- '40link u through the medium of a Substantially upended claims, referencebeing had to the accomshaped bracket member It. The lower end of thecylinder I3 is pivotally mounted for swinging movement at I'I to a baseor bracket I8 mounted 5 upon the floor I I. By virtue of the presence ofthe piston rod IB, it will be understood that when the piston I4 islocated at a point in the cylinder midway its opposite ends. the area I9below the piston is somewhat greater than the area 2li above Fig; 2 isan enlarged sectional view of t e valve to the Piston.

mechanism of the present invention, taken substantially along the line-2-2 of Fig. 1,1ooking in the direction of the arrows; Fig. 3 is aperspective view of a somewhat modified arrangement of a power unitconsisting of an The system includes an oil reservoir 2| and a pump 22adapted to be driven or operated by means of an electric motor mountedwithin a. casing 23, there being, of course, oil lines or reservoir withthe cylinder Il. Preferably the reservoir is above the valve unit asshown, to make for easier nlling. The motor 2l is of the reversible typecapable of being driven in either a clockwise or a counterclockwisedirection so that the pump 22 may likewise be driven in eithervdirection. 'I'he reservoir 2| communicates y througha pipe line or tube24 with a compensating or shuttle valve vunit shown as a whole at 2li.see particularly Fig.` 2. 'I'he oil pump 22 is connected by means of apipe line 28 to the left side of the valve 25 through the medium of aT-shaped coupling 21 to which is attached a .pipe line 28 which extendsthrough a base plate 20 mounted on the door II and is connected toanother pipe line ll which extends to and is tapped at li into the capor upper end lla of the cylinder.

A pipe line il extends'from the pump 22 to a T-coupling ll, similar tocoupling 21, and is tapped into the right side of the valve 2l. Anotherpipe line Il extends from the T-coupling through an opening in the floorplate 29 and is connected to a pipe line It which is tapped into orconnected at Il to the lower cap member or closure ilb oi' the cylinder.

The instrument panel l of the vehicle supports an electric switch 40controlled by a pull rod Il. This switch is electrically connected bywiring or cable 42 to av solenoid Il mounted upon the motor casing.

As described above, my improved hydraulic lii't mechanism or systemincludes means for connection to a frame member or linkage at one sideonly oi' a collapsible top. It will be understood that if it isdesirable to connect the hydraulic mechanism to a similar frame memberor linkage at tbe opposite side of the top. an additional cylinder lltis provided. The construction of the cylinder I". its piston, pistonrod, mounting means and the means for attaching the piston rod to thelinkage of the top is similar to that above described. As shown inFig..1, the pipe line 28 extends through an upright supporting bracketIt and is connected to a transversely extending pipe line 4l whichpasses through brackets 41 and member and to the floor respectively, andis connected to a pipe line 49 which extends to and is connected withthe upper end of the cylinder Ill. The pipe line 2B from the oppositeend or side of the valve structure 2l has a branch connection throughthe bracket 4l with a transversely extending pipeline l0, which pipeline extends through bracket 41 and plate 4l and is connected with apipe line Il which is tapped into the lower end or head Ilb of thecylinder by means oi' a tap or connection 52. It will be understood thatall or portions oi' the pipe line referred to may be in the form oi'ilexible hose or conduit, thus permitting freedom of movement oi' thecylinders Il and |30. Thus, when the motor 22 and pump 22 are driven ina clockwise direction, oil passes under pressure through the lines 28,2l and III t0 the upper end of the cylinder Il and at the same timethrough the lines 4l and I! to the upper end of the cylinder |30. 'I'hepressure of the oil or other iluid in the system when V.the motor andpump are being driven in a clockwise direction forces the pistons in therespective cylinders I3 and |30 downwardly to move the links or topframe members or bows downwardly toward the floor Ii to collapse the top(not shown).

Referring now particularly t0 Fis. 2, and with high pressure beingapplied to the chambersl above the pistons in the cylinders Il and |20,the parts of the compensating or shuttle valve unit 25 are in thepositions shown in this figure.

The valve structure 2l has a body portion 00 which, as shown merely byway of example, is of substantially T-shape having a transverse port orpassageway 8| and a connecting longitudinal passageway 82. It will beunderstood, however, that the body portion may be of any desired shape.Valve seats l2 and I4 are formed in the body at opposite ends of thepassageway or valve bore 62. 'I'he body is hollowed out at its oppositelongitudinal end portions to provide sockets o r chambers Il and 88.These chambers are internally threaded at 05a and a to receive andsupport threaded coupling nuts 6l and Il. Pipe lines 2| and Il areconnected with the couplings Il and 8l, respectively.

The shuttle valve assembly for each of the chambers It and I6 and thepassageway l2 is identical. As shown, the assembly at the lei't of Fig.2 comprises a poppet or disk valve Il mounted upon a stem 10 and heldagainst an abutment by spring 14. The valve l2 has one or more openingsor passages tta therethrough. The valve stem is supported in position bymeans of a compression spring 1I whose inner end engages the head onstem 'It and also engages the central portion of the valve Il. The outerend of spring 1i seats against the inner face of member Il and ismaintained in position by means of a series of fingers or projections 12on said member. l

The stem 10 also carries a relatively smaller disk valve or excesspressure relief valve 'Il which is normally urged toward the poppetvalve Il to close the openings or ports "a in the valve Il 40 by meansof a compression coil spring 1l, having Il attached to a body frame itsouter end seated against the valve 'Il and its inner end against awasher 1t adjustably held in position by a nut 16.

The valve assembly at the right of Fig. 2 comprises parts correspondingto those of the valve assembly at the left of this figure, namely, adisk valve 12 having one or more holes or openings 'la therethrough. Thevalve is mounted upon a stem l0' and is supported in position by acompression spring Il whose outer end engages and grips the head onstern I0 and also engages the central portion of the valve 1l. The innerend of the spring ll seats against the inner face of member 8l and ismaintalned'in position by means of a series of fingers or projections l2on said member.

The stem l0 also carries a relatively smaller excess pressure reliefdisk valve which, as shown herein, is held against the poppet valve 'Ilto close the openings 'lla formed therein, by means of a compressioncoil spring Il having its outer end seated against the valve Il and itsinner end against a washer l5 adjustably held in position upon the stemIl by a nut Il.

'I'he filling and bleeding of the system may be accomplishedsubstantially as follows:

The system is illled through the reservoir 2| by pouring fluid into thereservoir and running the pump 22 in one direction thereby sucking thefluid from the reservoir until the piston I 4 of cylinder I3 has reachedthe end of its stroke and no more iluid can be added. The pump is thenoperated in the opposite direction and the reservoir is filled asnecessary and fluid sucked from opposite direction several times untilall of the air is completely forced out through the relief poppet valves19 and 93. v

The operationv of the mechanism heretofore de'. scribed is substantiallyas follows:

The pull rod 4I on the instrument panel is actuated to operate theelectric switch 49 and the solenoid 43 to run the motor and drive thepump 22 in a counterclockwise direction. When the pump isv thus drivenor run in a counter-clockwise direction, the fluid is drawn from theupper end of the cylinder I3 and forced into 'the lower end thereof.However, as the volume or capacity ol the upper end of the cylinder isless than that of the lower end due to the presence therein of thepiston rod l5, a suction is-created in line 2B opening the poppet valveor flapper 69 and allowing fluid to be drawn from the reservoir 2il pastvalve il into' the line 29 and hence to the lower end of the cylinder tocompensate for this difference; The ilapper or poppet valve 19, ofcourse, is closed under the influence of its spring 8i and the pumppressure in coupling 34. When the line pressure of the lower end buildsup to a predetermined high pressure, the relief poppet tt starts'to opengradually until at any predetermined pressure such, for example, as 125pounds per square inch,'it hasopened suiilciently to by-pass enoughiluid through the holes 19a in the dapper 19 on the .pressure side ofthe pump. around the open flapper or poppet valve 99, to the suctionside of the pump 22 so that no further pressure can be built up and theline pressure will remain constant and never exceed thepredeterminerivvpressure setting.

When the pump 22 is operated in aciockwise direction the fluid is' drawnfrom the lower end of the cylinder I2 and forced into the upper endthereof. However, as the volume or capacity of the upper end ofthe-cylinder is less than that of the lower end due to displacement bythe piston rod, an excess or surplus of fluid is left in the'lower endofthe cylinder as the piston I4 starts to move downward under the pressureof the fluid builtup in line 26. The iiapper valve 69 remains closedunder .thisV pressure. This excess fiuid in thelower end of the cylinderis then put under compression by the movement downward of ,the 'pistonI4 and pressure is also built up in the lower end causing the 'iiappervalve 19 to also' remain 'closed as aresult of thepressure appliedltothe -uppr endv and the consequent movement of-thepiston. These pressuresboth increase'until-relief poppet 13 starts to open according to itsvpredetermined setting and allows this excess oilto by-p'ass throughholes 69a in "the'apper valve 99 and into the 'reservoir 2i. When allofthis excess fluid hasl byi-passed into the' reservoir, then line 9i`becomes the suction sidel of the pump and the by-pass at pop otrpressure (125 pounds per square inch) is as before, from the line 26 atthe pressure side through holes 99a inthe poppet valve or iiapper 69around the" open ilapper 19 to the line 3B at the suction side. ThusFigure 2 shows this condition after this excess liquid has beenby-passed into the reservoir, and reliefl valve 13 remains open and therelative suction in coupling .34 has opened valve 19. Thus in this formof the invention with .the main valves independent, the relief valves 13and 8l are essential to the operation, as it will be apparent that thepiston could not move if they were not present and there were nosubstantial leakage.

i The foregoing description relates to the operation of the device inconnection with a single cylinder I3. It will be understood' that when asecond cylinder, namely, the cylinder |30, is employed, theinterconnecting lines 46, 50, etc., as described above, function withtheir associated lines 26, 93, etc., and in connection with cylinder I3to actuate the pistons in both cylinders sivmultaneously.

It is to be understood that the reservoir 2i is used only as acompensatory and also a reserve supply reservoir. This reservoir'compensates for the diil'erence in volume between the upper end of thecylinder (or cylinders) rand the lower end thereof due to the presenceof and therefore volume or space occupied by, the piston rod i5.

In other words, Without the reservoir 2i, the volume of oil in the lowerend of the cylindei` would all have to be forced into the smaller spaceor volume at the upper end of the cylinder. This, of course, would beimpossible and the result would be a locking or binding of the oil pumpand the piston at 'some point in the stroke. Moreover, due to lthe'spaceoccupied by the piston it, there is not sufficient oil in the upper endof the cylinder I9 to completely fill the lower end thereof. However,the reservoir 2li also provides means for compensating for volumechanges of the oil due to the operating pressure and to the expansion orcontraction of said oil due to any temperature variation. In addition,the compensating reservoir permits filling of the system Without "thenecessity of external bleeding, that is, the

pressure balance valve, through the medium of the reservoir, isself-bleeding when the system is filled.

By virtue of the factthat the Vreservoir 2i is merely a compensatingreservoir for the slight changes in volume, and because of the fact thata slight vacuum or a few pounds of pressure in the reservoir would makenodifference to the operation of,r the system, due to the presence ofthe pressure balance valve, the reservoir can be made airtight andthusthe whole fluid system becomes a wholly sealed unit Referring nowparticularly to Fig. 3 of. the drawings, there is shown a somewhatmodified and preferred arrangement of the power unit constituting thepresent invention. In this form of the invention the electric motor andthe gear pump 9| for the fluid are arranged in tandem and preferablyin-a horizontal plane. The valve unit duit bands, straps, or members 96and 91 conncct the source of power to the solenoids and to the electricmotor. The switch 40, shown in Fig. l, is connected with'the solenoids93 and 94 through the medium of the wires or cables 42, seen also inFig. l.

` In other respects, the present arrangement of power unit is similar tothat of Fig. 1. there being a pipe line 2l leading to the upper oil orother fluid reservoir 2|. Pipe lines 2t and 3i, corresponding to similarlines shown in Fig. 1, leail nism embodying the present invention andinstallation, mounting, and service are easier, and the cost is reduced.

The valve mechanism shown in Fig. 4 is substantially like that of Fig. 2except that the poppet or disk-like valves 69 and 'Il are controlled oractuated by mechanical means as well as by fluid pressure in the system.That is to say, when one poppet is' held against its seat by fluidpressure. the other poppet is held'off its seat mechanically. With theparts yin the positions shownin Fig. 4. fluid pressure in the system isholdirig the valve 68 on its seat BI whereas the valve 19 issimultaneously held oif its seat Il by mechanical means. As shownherein, merely by way of example, thevalve stems 10 and Il have inwardopposed extensions 10a and 80a, respectively, which project inwardlybeyond the inner faces of the spring adjusting nuts 'It and Il and abutor engage one another. Thus, the valve stem 10, 10a mechanically holdsthe valve 'i9 oif its seat. With the parts in the reverse position ortoward the left of Fig. 4, with the valve 'l0 against its seat 64, thevalve stem portions 'I Ila. 80a will mechanically maintain the valve 09unseated or oil its seat 63. As will be seen from Fig. 4, and since thetwo valve stems 'l0 and 80 are long enough to hold one valve off itsseat when the other is closed, it will be apparent that when the valveunit itself (the two valves 8l and 19 and their stems 1li and 80) isheld in its normal or inoperative central position by the identicallight centering springs Il and 8l, the valves will be slightly oil theirseats and will thus permit la small by-pass leakage through the valvebore.

This feature is valuable since it permits a direct manual operation ofthe top when the pump is inoperative, as when the electric I'notorcannot be turned on or the like. For example, if the electric motor isenergized through the ignition switch, the top can be raised ii' theowner is away with the ignition key.

As will be apparent, the valve umts shown in Figs. 1 and 4 and havingtwo valve rods, one for each end of the valve bore, will permitindividual adjustment or removal and insertion of the two valves andtheir stems. If desired, a single common valve stem maybe employed forboth of the valves 89 and 10, which if used, would supplant abuttingseparate valve stems 10, 10a and 80, 80a. In that event, a single springbetween the relief poppets 1l and 83 would take the place of theseparate springs 14 and I4. It will be understood that if a single valvestem were used, it would be of .the proper length so that when one ofthe poppet valves was seated, the other valve would be oil.' its seatand here also there will be the by-pass leakage in inoperative positionto permit direct manual operation of the top and consequently of thedirect connected piston motor. As will be apparent, both here and in theform of Fig. 4, thev centering springs will aid in holding the valvesslightly of! their seats to permit slow manual operation since thevalves will not be closed until there is an appreciable pressure sidevalve. v

This single valve stem form of the valve unit is shown in Fig. 5, whichis identical with and has; referencev 'characters similar to, Fig. 4,except for the single spring 14 for the relief valves, thc single valvestem 10', and the omission -of parts 1l, il, and separate abutting stemparts Na and Ita. It will also be seen that Fig. 5 shows both the valvesBl and 'Il held oif their seats` in their neutral leaking position.

When the valve mechanism of Fig. 4 is employed, the filling and bleedingoi' the system would be accomplished substantially as follows:

The system is filled through the reservoir 2| by pouring fluid intothelreservoir and runninz the pump 22 in one direction thereby suckingfluidA from the reservoir until the piston i4 has reached the end of itsstroke and no more fluid can be added. The pump 22 is then run in theopposite direction with the reservoir kept full and the oil or otherfluid is sucked from the reservoir and other end of the cylinder I3until the piston has reached the end 'of its stroke in the oppositedirection.

. Bleeding m'ay be accomplished in two ways in the present System.

The pump is `first operated several times, iirs't in one direction andthen in the other direction. When the pressure poppet or napper valvecloses, the other poppet is mechanically held open by the contactingstems 'Illa and 30a so that the supply side vof the valve mechanism orline is always open to the reservoir 2| and to atmosphere. Accordingly,any air in the system is free toescape from the side not under pressureduring the operation of the unit and the movement of the piston.

Additional bleeding is accomplished through the relief poppets 13 and Ilwhich open gradually under pressure and allow the air to be forced out.

'I'he operation of the system when embodying the valve mechanism of Fig.4 or of the common valve stem form is substantially as follows:

When the pump 22 is run or operated in a.v

'counterclockwise direction, the fluid is drawn from the upper end ofthe cylinder I3 and forced into the lower end thereof. Thus pressure inline I5 closesthe poppet valve I8 and'l opens valve 88 as the flowincreases. The reservoir is then mechanically placed in communicationwith the upper end of the cylinder line so that the needed extra fluiddue to the piston rod `displacement in theupper end of the cylinderdirection the fluid is drawn from the lower end of the cylinder I3 andforced into the upper end thereof. Thus pressure in line 28 similarlycloses the poppet or iiapper valve 69 and opens the valve or i'iapper10. The reservoir 2| is then mechanically communicated with the lowerend of the cylinder line so that the excess iiuid in the bottom of thecylinder Il, due to the displacement of the piston rod in the upper endof the cylinder. is free to ilow into the reservoir during the operationof the unit and the movement of the piston I4. l

The relief poppet 13 functions in exactly same manner as in Fig. 2.

' 'I'hus it will be apparent that in these other the forms of theinvention, in which the main valves 69 and 19 are not independent butinteract so that when one isclosed it holds the other oif its seat, therelief valves 'I3 and 83, While not essential to operation as in the rstform, are highly desirable to not only limit excess pressures in themotor, as from thick oil on Va cold morning and to permit the use of anexcess powered electric motor and pump, but, more important, to providea safety feature whereby the rat of motion' of the convertible top andparticularly the force pushing it are limited so that it is notdangerous (as to children) and, in fact, may be stopped by hand if theswitch cannot be reached. Obviously, these advantages may also berealized in the first form.

It is to be understood that the operation is the same as above describedwhen two cylinders I3 and |30 are employed.

It will be understood that the hydraulic power system embodying thepresent invention can, of course, be operated from any convenient formof power other than the electric motor described herein. If the sourceof power could not convenientlybe reversed, a valve could be installedin the pressure and suction lines in the vicinity of the pump 22 toreverse the flow of fluid in these lines by means of a moving plungerinstead of by means of reversing the pump itself.

I claim:

l. In a hydraulic lift mechanism for raising or lowering a convertibleautomobile top or the like, in combination, a reversible electric motor,

a liquid pump rotated in either direction thereby, a double actingcylinder and piston motor to raise or lower said top and having a pistonrod at one end only so that unequal volumes are displaced at the twoends of the cylinder, two conduits connected to carry liquid fromopposite sides of the pump to opposite sides of the piston,

a compensating and replenishing valve unit comprising means defining avalve bore connected between said conduits, a liquid reservoir connectedto the intermediate portion of said bore and forming a sealed systemwith said conduits, the pump, the valve, and the cylinder, said borehaving at each end a valve seat and a cooperatf ing valve member to beselectively seated by liquid ilow from the pump outlet, rod meansbetween said valve members and having a length slightly greater than thespacing between said valve seats, and centering spring means normally orin inoperative position holding said two valve members slightly offtheir seats to permit manual lifting of the top and consequent actuationof the piston. A

2. In a convertible automobile top mechanism, a wholly sealed hydraulicactuating mechanism comprising a double acting piston motor having onlyasingle piston rod, a reversible liquid pump, a reversible electricmotor driving said pump, conduits between said pump and motor to operateit in either direction, a liquid reservoir, and a shuttle valve unitincluding movable valve means actuated automatically and only by theliquid flow and pressures for selectively connecting said reservoir tothe then low pressure conduit for operation of the top and motor byliquid now through said by-pass leakage passage, and said movable valvemeans including, as va part thereof and moving and cooperatingtherewith, resiliently biased excess pressure relief valve means tolimit the pressure in either4 end of said piston motor in eitherdirection of its travel by venting it`to the other end of the cylinderand to the reservoir to thus permit the use of an excess poweredelectric motor and pump with safety to careless passengers.

3. In a mechanism for convertible automobile tops or the like, ahydraulic actuating system comprising a double acting piston motorhaving a single piston rod to directly raise or lower said top. areversible electric motor, a liquid pump means connected to operatesaid-motor, a reversing electric switch as the sole control for saidelectric motor and pump. a compensating shuttle valve unit operatedautomatically and only by the pressures due to liquid flow and connectedacross said pump connections, a liquid reservoir positioned above saidvalve unit and connected to said valve unit to be selectivelycommunicated with the then low pressure side of the pump durlng pumpoperation, said shuttle valve unit including resilient valve centeringmeans and cooperating means to provide upon non-operation of the pumpmeans a by-pass leakage through said valve unit in its centeredinoperative position to permit direct and slow manual actuation of thetop and its motor piston.

4. In a mechanism for convertible automobile tops or the like, anhydraulic actuating system comprising a double acting piston motorhaving a sin'gle piston rod to directly raise or lower the top,reversible liquid pump means connected to operate said motor, a liquidreservoir, and a motor Volume change compensating shuttle valve unitcomprising means den'ning a valve bore connected across the pump liquidconnections, a connection from the reservoir to the intermediate portionof said bore, an outwardly facing valve seat at each end of said bore, apair of coaxial and normally abutting valve rods in said bore, eachcarrying at its Outer end a disk valve mount ed thereon to be movableback and forth thereeither direction of piston motion, said valve unit vwith and to be selectively closed against the valve seat only by thepressure from the then high pressure side of the pump so that it actsdirectly on its valve rod and through the abutting valve rods to pushthe other disk valve toward open position and to lift it off its seat,the length of said abutting rods and the spacing between said diskvalves being greater than the distance between said valve seats, so thatwhen said rods andl their valves are centered both valves are off theirseats to provide a through or by-pass leakage passageand a centeringspring in each end of the valve bore acting-aaginst each disk valve toautomatically return them to and normally hold them in centeredpositions 01T their seats during non-operation of said pump means or lowrates of liquid flow and pressures to thus permit slow manual operationof said top and said piston motor by liquid flow through said by-passleakage passage.

5. In a raisable or lowerable automobile convertible top mechanism orthe like, an hydraulic mechanism comprising a double acting piston motorhaving only a single piston rod, a reversible ilow liquid pump, areversible electric motor driving said pump. conduits between said pumpand motor to operate it in either direction in accordance with thedirection of rotation o! said electric motor, a compensating liquidreservoir, and a compensating valve unit for selectively connecting saidreservoir to the then low pressure conduit for either direction ofpiston motion to compensate for the difference in volumes due to thesingle piston rod, said unit including two independent and unconnectedoppositely acting valves resiliently biased to their closed positionsand r1o

